Automatic transfer switches must be periodically tested and maintained to be sure that they will operate automatically when needed. Automatic transfer switches have become especially important since more and more sectors of the economy are purchasing emergency generator sets (i.e., computer installations, hospitals, etc.). It is essential that the reliability of automatic transfer switches be demonstrated without difficulty. Heretofor the testing of automatic transfer switches has required the use of a bypass switch assembly to temporarily connect the electrical load to an available source of power, an isolating switch assembly to isolate the automatic transfer switch from the bypass switch, and a test load. In addition, cable interconnects and interlocks have been required to assure safety while testing. Designs for the automatic transfer switches are illustrated in U.S. Pat. Nos. 3,283,089; 3,778,633; and 3,936,782.
Attempts have been made to improve the state of the art. One such arrangement is shown and described in U.S. Pat. No. 3,697,709, wherein the bypass switch and isolation switch functions were combined.
Another attempt to improve the prior art (illustrated by U.S. Pat. No. 4,157,461) eliminated the necessity for a separate isolation switch or switches by using sliding contacts of varying lengths to perform the isolation function (i.e., a set of long contacts to maintain power to the transfer switch for testing and a set of short contact to disconnect or isolate the electrical load from the transfer switch) while the power source and load were connected together by the bypass switch. Mechanical means were provided to interlock the various elements (i.e., to prevent isolation of the load from the automatic transfer switch, if the bypass switch was open, and to prevent switching of the bypass switch to connect the load to a power source different than that to which the automatic transfer switch was connected). Other improvements are certainly in order considering the importance of these switches.
It would be particularly desirable to improve the manner in which the automatic transfer switch is tested. A simple fool-proof procedure and preferably a test scheme incorporating automatic interlocks would be welcomed by the industry. This is especially true since these switches are being used by many industries and service organizations which do not routinely maintain a staff of skilled electricians and test engineers.